def p(n):
    for i in range(2,n):
        if n%i ==0:
            return False
        return True




x = int(input())

prost = [i for i in range(1,10000) if p(i)]
m = 0
if 1000<=x<=9990:
    for i in prost:
        if str(i) in str(x):
            m = max(i,m)
print(m)
